The present invention pertains to a method for handling data transmission frames of variable length with a channel controller and for writing them to a cyclic buffer memory wherein the data transmission frames consist of a header field containing at least control data, a data field and an end field of variable length.
In the interface between the PCM and GSM networks, communication takes place by using TRAU frames which are defined in the GSM specifications and which possess the following properties:
- recurrence interval 20 ms
- contains data relating to one traffic communication (voice/data channel)
- reserves one PCM time slot
- transparent to the PCM channel
- contain 320 bits of control information and data.
In order for the channel decoder receiving the TRAU frames to be capable of operating in compliance with the GSM specifications, the frames must arrive at the channel decoder correctly phased. Owing to time slips of clocks, maintenance of the correct phasing requires that the TRAU frame be shortenable or extendable by four bits, i.e. the length of the TRAU frames be in fact 320.+-.4 bits. The channel decoder is informed of whether the TRAU frame concerned has been extended or shortened by control bits located at the very beginning of the frame.
The situation described above is problematic, because TRAU frames may nave a length varying four bits in both directions while the length of the buffer section is invariable. Several extended or shortened TRAU frames then cause the frame to slip within the buffer section, whereby the controller interrupts the channel decoder at the wrong time, either too late (TRAU frame shortened) or too early (TRAU frame extended), i.e. when the buffer section is full. The only right moment for interrupt would be when the buffer has a complete TRAU frame. For instance, in a GSM base station slip causes unnecessary delay in the speech signal.